Here are some wonderful tools for Middle Grades (3-6) aged children (we'll be adding more, and we take suggestions, too):

Reading:
UEN.org - Great interactive tools that supplement class activities
Into the Book - great activities that support reading comprehension
Lit 2 Go - Hundreds of classic books available as audio selections, many with activities

Math:
The National Library of Virtual Manipulatives - Pre K-12 Algebra, Geometry and More\
Visual Math - online math lessons
http://multiplication.com/ - Great Tools for learning multiplication
Timez Attack: An amazing, free multiplication game http://www.bigbrainz.com/index.php?PARTNER=krimsten

Thousands of excellent how to videos and lesson ideas:
www.Teachertube.com

Cool worksheets (old school still works!) http://www.eslprintables.com/